Lean burn compression ignited internal combustion engine technology, such as diesel, has the capability to provide improvements in fuel efficiency over a spark ignited internal combustion engine. However, new exhaust aftertreatment systems may be required to meet future oxides of nitrogen (NOx) and particulate emission standards.
One such system is the selective catalyst reduction (SCR) technology, such as hydrocarbon-SCR and urea-SCR. Hydrocarbon-SCR technology relies on hydrocarbons to selectively reduce the NOx in a lean, i.e. oxygen rich, environment with a specialized catalyst. Urea-SCR technology injects an aqueous urea solution into the exhaust stream, which then decomposes to ammonia that operates to reduce NOx over a catalyst. The urea-SCR technology requires periodic refilling of an on-board urea tank.
Another system is the lean NOx trap, or LNT. The LNT is an aftertreatment technology that employs catalyst devices, which include oxides of alkaline metals such as barium and cerium. The LNT catalytically oxidizes nitric oxide (NO) to nitrogen dioxide (NO2), which is then stored in an adjacent chemical trapping site as nitrate (NO3). The conversion efficiency of the LNT has been demonstrated to exceed 90%.
Once a quantity of NOx is absorbed by the LNT, a regeneration process is required to chemically reduce the nitrate to nitrogen to allow the LNT to trap or absorb additional NOx molecules. The conventional approach to regenerating the catalyst of an LNT is to temporarily introduce reducing agents such as hydrogen, carbon monoxide (CO), and hydrocarbons (HC) to the LNT. The reducing agents are often formed by operating the lean burn internal combustion engine at rich air/fuel (A/F) ratios. However, the products of rich combustion are not efficient reducing agents and as a result, the LNT catalyst must be over-regenerated, i.e. supplied with a large amount of reducing agents, to achieve high NOx conversion.
Hydrogen is an effective reducing agent to regenerate accumulated NOx within the LNT. There are several potential methods for supplying hydrogen on-board a vehicle to regenerate the LNT. One method would be to refuel a hydrogen storage tank from an external refueling station. A second method would be to equip the vehicle with an onboard reformer to convert fuel into hydrogen and carbon monoxide. This approach delivers a gas mixture of hydrogen, carbon monoxide, and nitrogen. The presence of carbon monoxide may inhibit the regeneration reaction at low temperatures, i.e. below 250° C.
With a typical LNT, having a Barium/Potassium formulation, the NOx reduction reaction proceeds within a temperature window of about 150° C. to 500° C. and NOx is desorbed without reduction above 600° C., when using hydrogen as the reductant. Furthermore, a desulfation step is required to maintain NOx absorber efficiency when operating the lean burn internal combustion engine with fuel containing an amount of sulfur. The desulfation step requires exhaust gas temperatures, at the LNT, greater than 600° C. for several minutes.
Also of concern is the reduction of particulate matter emissions from the lean burn internal combustion engine. Particulate matter emission is filtered from the exhaust stream by a device known as a diesel particulate filter, or DPF. The DPF typically filters the exhaust stream via wall-flow filtering. The DPF requires a regeneration step to maintain high filtering efficiency and to decrease associated exhaust backpressure on the engine. The regeneration process of the DPF requires temperatures greater than 600° C. in lean (oxygen rich) exhaust operable to burn the particulate matter trapped within the DPF. An alternative option to the DPF is the diesel particulate NOx filter, or DPNF, which is a catalytic combination of an LNT and a DPF. The DPNF utilizes NOx to continuously oxidize particulate matter from the filter.